1. Field of the Invention
The present invention generally relates to a steam-cooling type gas turbine and more particularly to an improved cooling structure therefor which can effectively prevent leakage of coolant steam.
2. Description of Related Art
A combined cycle power plant comprised of a combination of a gas turbine plant and a steam turbine plant is designed such that the gas turbine is operated in a high temperature region with the steam turbine operated in a low temperature region in a sharing mode to efficiently recover thermal energy for effective utilization thereof. In recent years, this type of power generating system has been attracting public attention from the standpoint of high thermal efficiency.
In conjunction with the cooling of moving blades of the gas turbine in the combined cycle power plant such as mentioned above, it is noted that in the present state of the art, a steam cooling system is replacing an air cooling system. Parenthetically, in the steam cooling system, a part of steam generated in the steam turbine is extracted to be led to the gas turbine for cooling the moving blades thereof, and the steam having a temperature raised after cooling of the moving blades of the gas turbine is recovered to be fed back to the steam turbine cycle to achieve more effective utilization of thermal energy. Thus, it is expected that the steam cooling system can contribute to improvement of the operation efficiency of the combined cycle power plant. For this reason, the steam cooling system is attracting attention in these years.
FIG. 13 is a sectional view showing schematically a portion of a typical one of the conventional steam-cooling type gas turbines. In the figure, reference numerals 50 and 51 denote casings of a compressor and the gas turbine, respectively, wherein a rotor 60 having a large number of moving blades mounted therearound in rows and designated representatively by reference numerals 71, 72 and 73 is disposed within the turbine casing 51. A high temperature combustion gas discharged from the associated combustor is introduced through a combustion gas passage 52 into spaces defined between stationary blades 83; 84; 85 disposed on the inner wall surface of the turbine casing 51 and the moving blades 71; 72; 73 to undergo expansion, to thereby force the rotor 60 to rotate.
On the other hand, there are formed in a disk 61 of the rotor 60 a plurality of circumferentially distributed steam passages 63 which extend axially through the disk. The coolant or cooling steam 80 is introduced into the individual steam passages 63 from a steam inlet 65 disposed in a turbine shaft 64 to flow through other passages 62 formed similarly in the disk 61, wherein a part of the cooling steam 80 enters a cavity 66 and hence flows into moving blades 72 of the second stage by way of steam feeding passages 67 to cool the second-stage moving blades 72. Thereafter, the cooling steam 80 reaches a cavity 69 by way of steam recovering passages 68. Further, another part of the cooling steam 80 flows into steam feeding passages 91 by way of a cavity 90 to enter the moving blades 71 of the first stage for cooling the interior of these moving blades. Thereafter, the steam reaches the cavity 69 by way of steam recovering passages 92. Thus, within the cavity 69, the flows of the steam recovered after cooling of the first-stage moving blades 71 and the second-stage moving blades 72 join together to enter another cavity 93. Thereafter, the steam flows through a center passage of the rotor 60 to be recovered at the side of the turbine shaft 64. Additionally, a part of the steam flowing through the steam passage 62 is supplied to the compressor 50 as well by way of a cavity 94 to be used for cooling the compressor 50. At this juncture, it should be mentioned that each of the steam passages 62 and 63 may defined by a pipe.
As is apparent from the above description, the conventional steam cooling system is so designed that the steam of low temperature and high pressure flows through the passages implemented internally of the rotor. Consequently, there are many locations where the steam leakage may occur to the external low-pressure environment, giving rise to a serious problem in the steam cooling system with regards to prevention of the leakage of the feed steam, i.e., steam to be fed to the moving blades of the gas turbine.
FIGS. 11 and 12 are sectional views showing fragmentally another example of the conventional gas turbines in which the steam cooling system is adopted. More specifically, FIG. 11 shows a rear portion of a fourth stage of moving blades in the conventional gas turbine. Referring to the figures, a rear disk (journal bearing) 102 is mounted onto a fourth-stage disk 100 through interposition of a seal disk 101, wherein an outer rotatable shaft 103 and an inner rotatable shaft 108 are mounted on the rear disk 102 so that the fourth-stage disk 100 can rotate together with the outer rotatable shaft 103 and the inner rotatable shaft 108. Rear end portions of the outer rotatable shaft 103 and the inner rotatable shaft 108 are enclosed by a stationary housing 104 which is disposed in opposition to both the rotatable shafts by means of a bearing portion 105 serving as a seal portion for the outer rotatable shaft 103 and a bearing portion 107 serving as a seal portion for the inner rotatable shaft 108. A high-pressure chamber 106 is defined between the rear end portion of the outer rotatable shaft 103 and the housing 104, while an annular steam passage 109 is defined between the outer rotatable shaft 103 and the inner rotatable shaft 108.
In the steam-cooling type gas turbine of the structure mentioned above, feed steam 120 (see FIG. 12) flows through the steam passage 109 from the high-pressure chamber 106 to enter an annular high-pressure chamber 110 from which the steam flows into a cavity 112 by way of a passage 111. From the cavity 112, the feed steam is introduced into the moving blades of the first and second stages (not shown either) via relevant passages (not shown) provided in the fourth-stage disk 100. The steam having done work of cooling the moving blades is recovered as the recovery-destined steam as indicated by an arrow 121 by way of a passage (not shown) formed in the inner rotatable shaft 108.
FIG. 12 is an enlarged view of the rear disk 102 shown in FIG. 11. Referring to FIG. 12, the portion of the rear disk 102 which lies adjacent to the seal disk 101 is heated by the recovery-destined steam 121 having a raised temperature and has a higher temperature than the other portion of the rear disk 102. Consequently, the portion of the rear disk 102 located adjacent to the seal disk is subjected to the influence of thermal expansion, as a result of which a disk coupling bolt 113 tends to be tilted under tension, as indicated by an arrow in FIG. 12. Consequently, a part 120a of the feed steam 120 will leak to the exterior through a clearance making appearance due to the tilting of the disk coupling bolt 113.
As is apparent from the foregoing description, in the typical steam-cooling type gas turbine known heretofore, the steam extracted from the steam turbine is introduced into the moving blades of the gas turbine via the disks from a plurality of steam passages provided internally of the rotor for cooling the moving blades. The steam heated to a high temperature after cooling of the moving blades is collected into the steam collecting cavity and fed to the center passage formed in the rotor, from which the steam is recovered to be fed back to the steam turbine. Owing to such cooling scheme effective utilization of the steam can certainly be achieved. However, with the conventional steamcooling system in which the steam of low temperature and high pressure is fed along the peripheral portion of the rotor, there exist many locations where the steam can leak to the ambient or environmental areas in the course of flowing toward the moving blades, which in turn means that an increased number of seal portions have to be provided in order to prevent such leakage of the feed steam. In other words, in the conventional steam-cooling type gas turbine, it remains as an important problem to be solved by what measures the leakage of the feed steam of high pressure to the environment of low pressure can be prevented.
Furthermore, in the conventional steam cooling system shown in FIG. 12, the rear disk (journal bearing) 102 is heated to a high temperature because the rear disk 102 is exposed to the recovery-destined steam 121 passing through the coupling portion interposed between the rear disk 102 and the rotor disk, as a result of which the rear disk or journal bearing 102 undergoes thermal deformation such that the outer peripheral portion thereof becomes apertured or opened to allow a part 120a of the feed steam 120 to leak therethrough. Besides, there arises a problem that an excessively large tensile stress may be induced in the disk coupling bolt 113 due to the thermal deformation mentioned above. Additionally, since the feed steam 120 flows at the radially outer side of the recovery-destined steam 121, steam leakage may occur through the bearings 105 and 107 serving as the stationary seals for the outer rotatable shaft 103, which will of course incur reduction in the amount of the feed steam to be supplied to the moving blades.